by Courtney Llewellyn
Much like a superhero, as a nursery owner you know that once you defeat one villain, another pops up – such as the redheaded flea beetle. A native chewing insect, it exists from Maine to Florida and from Texas to Montana. Nurseries across the country have reported damage from the buggers. Fortunately, Brian Kunkel, Ph.D., an Extension specialist for ornamentals IPM at the University of Delaware, shared insight on management options for the pest in a recent Horticultural Research Institute webinar.
“It’s difficult to keep populations suppressed and from causing damage,” Kunkel said of redheaded flea beetles (RHFBs). That’s because they feed on many different families and species of plants, including Sedum, Joe-Pye weed, Salvia, Weigela, asters, blueberries, hibiscus, hydrangea, Pennsylvania smart weed and – new this year – Humulus, Chelone, Alchemilla, Cephalanthus and Physocarpus.
RHFBs chew holes through leaves or create brown divot marks. They have no preference for the tops or bottoms of leaves. Their head capsules are reddish-brown; the tail ends have fleshy appendages that clearly indicate they’re flea beetle larvae. They can be seen using a hand lens.
Kunkel said one of the problems with the beetle is that the larvae hatch from eggs laid on top of the soil or in the soil – they’re not 100% sure where it occurs – and larval feeding encompasses some of the roots of the plant. The plant can look fine but be heavily infested. He said you need to scout for the larvae – take the root ball out and inspect it. The larvae look a lot like root hairs and are usually around 1 cm long. He started finding them as early as 242 – 370 GDD, while plants were in bloom.
One major issue is how long RHFBs are active. The first adult generation becomes active between GDD 517 – 1,028 (when hydrangea are in bloom to full bloom). Larvae are active as the second generation between 1,570 – 1,860 GDD (with Hosta in full bloom and Miss Molly butterfly bush in late bloom). The next adults pop up at 1,878 – 2,318 GDD, based on Kunkel’s field observations. Eggs are laid between November and April, with larval feeding lasting about three weeks, but he has heard of feeding activity all the way into December in Virginia.
The good news is that nematodes have been helpful in controlling RHFB. Kunkel said they should be applied early in the morning or close to sunset to avoid dehydration and UV exposure. Entomopathogenic fungi can also be utilized, but they need high humidity in the substrate. (They also can be affected by contact and UV exposure.)
“We found we were fairly successful with nematodes in nursery tests,” Kunkel said. However, he added that it seems difficult to have long-term residual control with the products currently available.
While entomopathogens (nematodes and fungi) have potential, more work is needed to determine their viability and timing. In chemical trials, Mainspring appeared to protect foliage if it’s applied prior to adult beetle flight and may last up to 28 days. Neonicotinoids such as Safari, Flagship, Discus and Tristar reduced foliage damage and performed well in some trials against larvae and adults. Multiple products were tested by Kunkel’s team, but few provided consistent reduction in damage or mortality for more than a few days. “It’s all about finding something that fits into your operation and your budget,” he said.
Until better control options are found, growers need to focus on where RHFBs might be coming from – pots, weed-mats or even neighbors. Additionally, you need to pay attention to moisture levels. The beetles have been found in cranberry bogs, and they like wet, moist conditions.
“We request that anybody who is dealing with this, let Extension, ag associations and your state government know about this,” Kunkel said. “Funding needs to occur to study the problem to take advantage of windows of control.”